Two positive angles whose measures have a sum of 90^(∘).
Now, let's consider the given diagram.
There is one angle which is marked with a square — ∠ CDA. This means it is a right angle and its measure is 90 ^(∘). ∠ CDA is the sum of angles ∠ ADB and ∠ BDC, so the sum of measures of these angles has to add up to 90 ^(∘). This means that they are complementary.
